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8.1 Respiration
adapted from John Burrell http://click4biology.info
In the process of respiration, energy stored in organic food molecules is transferred to the molecule Adenosine
Triphosphate ATP. ATP in turn supplies the energy for metabolic processes in the cell. Synthesizing ATP uses
a series of linked oxidation and reduction reaction.
8.1.1 Oxidation and Reduction reactions
Oxidation Typically involves loss of energy
Reduction Typically involves a gain of energy
Oxidation and reduction reactions are always paired. In a ‘redox’ reaction, the substance that is oxidized,
reduced another substance, and vice versa. Larger food molecules are oxidized: They loss energy through the
breaking of a bond and this energy is in turned captured by another molecule, which is reduced. The energy
lost from the oxidation of organic food molecules can be absorbed by the reduction reaction that forms ATP.
Oxidation and reduction reactions can be recognized through characteristic chemical events
Oxidation
Reduction
May involve a loss of electrons
May involve a gain of electrons
May involve a gain of oxygen
May involve a loss of oxygen
May involve a loss of hydrogen
May involve a gain of hydrogen
1. What are four characteristics that may be seen in an oxidation reaction?
2. What are four conditions that might occur in a reduction reaction?
3. What are the paired oxidation and reduction reaction called?
8.1.2 Glycolysis Location: Cytoplasm of all cells
Outline: Oxidation of Glucose (6 carbons) to two Pyruvate (3 carbons) is coupled to the reduction of
ADP to ATP.
In the following models the hydrogen and oxygen atoms are not shown. The models show the number of
carbons in each molecule not the structural formula
The first stage actually begins by phosphorylating glucose at both
ends to make a hexose diphosphate.
The phosphate groups allow a stronger interaction between the
hexose sugar and its enzyme.
This stage involves the breaking of the hexose diphosphate into two
triose phosphate molecules.
The triose phosphate is an intermediate in many biochemical
reactions.
The phosphate group allows the sugar to form stronger interaction
with the next enzyme in the pathway.
This is the main oxidative stage of glycolysis which results in the
formation of ATP and NADH + H+
Each triose phosphate is oxidized to a 3 carbon molecule called
Pyruvate
Each triose phosphate has hydrogen removed to reduce one NAD+ to
NADH
Each triose phosphate adds a phosphate to Adenosine Diphosphate
reducing this to ATP (substrate level phosphorylation) Note that each
triose phosphate releases enough energy for the formation of two ATP
glycolysis and Energy
Energy
Budget:
- 2 ATP
+4 ATP
+ 2 NADH
8.1.3 Mitochondria: Location of aerobic respiration
Pyruvate, the product of glycolysis can be further oxidized to release more energy. This process takes place in the
eukaryotic organelle the mitochondria. Cells that need a lot of energy have many mitochondria (liver cell) or can develop
them under training (muscles cells)
8.1.4 Aerobic Respiration
Stages in the Aerobic respiration:
Link Reaction: Pyruvate is transported into the matrix of the mitochondria
Krebs cycle: carbon fragments (C2) are progressively decarboxylated to yield ATP and reduced coenzymes
Electron Transport System: reduced coenzymes are used to generate more ATP (see 7.1.5).
Link Reaction: Pyruvate (3C) is transported to the matrix of the mitochondria
A large Co-enzyme A joins with the 3 carbon fragment pyruvate.
Pyruvate is decarboxylated removing a single carbon as CO2
The remaining fragment is an Acetyl group and temporarily forms
Acetyl CoA.
NAD+ is reduced to NADH + H+.
Acetyl (2C) is already transported into the matrix.
The Krebs Cycle: oxidative decarboxylation of the C2 Acetyl group. This cycle has been broken down into 4 steps. The
carbons from the original glucose molecule are shown in purple and those of mitochondria molecules in blue.
Acetyl CoA joins with the C4(acceptor)group
CoA is released to transport more pyruvate into the matrix
a C6 fragment is formed (citric acid)
C6 (Citric Acid) is oxidative decarboxylated.
A C5 group is formed.
The Carbon is given off as CO2
NAD+ is reduced to NADH + H+
The C5 fragment is oxidized and decarboxylated further to a C4
compound.
Again the carbon removed forms CO2.
NAD+ is further reduced to NADH + H+.
The final stage in the cycle has the C4 acceptor regenerated.
There is a reduction of NAD+ to NADH + H+.
FAD (Coenzyme)is reduced to FADH2 .
ADP is reduced to ATP
Krebs Cycle and Energy
Energy Budget
per pyruvate
Link reaction
1 CO2
1 NADH + H
Krebs Cycle
2 CO2
3 NADH + H+
1 FADH2
1 ATP
1
8.1.5 Oxidative phosphorylation and Chemiosmosis Theory
On the inner membrane of the mitochondria (Cristae) there are membrane proteins.
The oxidation of reduced coenzymes (NADH and FADH2) allows these membrane proteins to pump protons (H+) into the
space between the outer and inner mitochondrial membranes.
These H+ form a high concentration (low pH) within this space. They diffuse back to the matrix through a channel in a
membrane protein called an ATP synthetase.
This flow of H+ through the ATP synthetase drives an enzyme reaction that brings about the phosphorylation of ADP to ATP.
The following sequence of diagrams breaks down this dynamic process down into a number of stages. There are a
number of membrane proteins involved in this process. Only a few of these proteins are shown and then only to allow
specific reference to the diagrams.
Oxidation of NADH ( coupled to ADP phosphorylation)
The membrane shown is only the inner mitochondrial membrane
folded into the cristae.
The NADH + H+. is oxidised and the reduced proteins transport
H+from the matrix into the space between both mitochondrial
membranes.
The electrons flow along the proteins reducing each one in turn.
For each NADH + H+., 3 Moles of H+ are pumped into the
space.
Oxygen supplied by the respiratory/ circulatory system acts as
the final H+ acceptor forming water.
Oxidation of FADH2 (coupled to ADP phosphorylation)
The FADH2 is oxidized and the reduced membrane proteins
pump H+ into the space between the mitochondrial membranes.
The H+ diffuse back to the matrix driving the ATP Synthetase to
produce ATP.
One FADH2 produces two moles of hydrogen ions.
Again, the H+ are accepted by oxygen to form water
ATP Synthetase
There is a high concentration of H+ in the space between the
membranes
The H+ diffuse down a concentration and charge gradient
through a channel in the ATP synthetase protein.
This ion flow causes a rotation in the head of the ATP
synthetase which drives together ADP and Pi in a
phosphorylation.
Each NADH + H+. produces 3 ATP
Each FADH2 produces 2 ATP.
Oxidative Phosphorylation Energy Budget (The following is based on one mole of glucose)
Stage
Number of NADH
+ H+.
Glycolysis
2
Link
2
Krebs
Sub-Totals
6
ATP
Synthetase
Number of FADH2
Number of ATP directly
made
(4-2)= 2
2
10
2
30 ATP
4
4
Total = 38 moles ATP
Efficiency of Respiration
How much of this energy is transferred to ATP molecules in the
process of aerobic respiration
How efficient is this transfer in a cell?
What happens to lost energy?
If a mole of glucose is combusted in a calorimeter then it
releases 2870 Kj Mol-1
The formation of one mole of ATP requires approx 31 Kj mol-1
During aerobic respiration as detailed above 38 mole of ATP are
formed from one mole of Glucose.
38 mole of ATP capture 31 kj X 38 = 1178 Kj
Efficiency : 1178/ 2870x 100 = 41 %
Lost Energy: approx 60% of the energy is lost mostly as heat
(important for homeotherms)
8.1.6 Mitochondria Structure and Function:
There are a number of structural adaptations that can be seen in the mitochondria that improve the efficiency of aerobic
respiration.
Structure
Function
The folded inner
forming cristae
membrane
Space
between
membranes
double
Matrix
Increase surface area for the
electron transfer system
Allows the accumulation of
protons
region isolating Krebs cycle
enzymes
Adaptation to Exercise
This electron micrograph of a muscle cell shows some
features of adaptation to aerobic exercise training.
Increased number of mitochondria per cell
Increase in the concentration of Krebs cycle enzymes in
the matrix
1.7 Fat Metabolism
Triglyceride fats can also be metabolized as a source of
energy.
The triglyceride is digested to its fatty acids
Co-enzyme A attaches to the carboxylic acid end
and an acetyl (2C) group is hydrolyzed. This occurs
between the beta and alpha carbon, hence beta
oxidation.
The acetyl CoA then moves into the Krebs Cycle
Various co-enzymes are reduced including FAD that
will then transfer to the Electron Transfer system.
The Fatty acid is further digested by the same
process to yield many acetyl (2C) groups